For the last three decades, rotomolding (or rotational molding) of plastics has received much attention because of the low machinery cost, simple tooling, and low level of waste involved. Rotomolding is currently one of the fastest-growing processes in the plastic industries, expanding at an annual rate of 25 to 30%.
A detailed discussion of the rotomolding process and specific plastics resin materials, is referred to in the article appearing in Modern Plastics Encyclopaedia (1995) at page D171, the disclosure of which is hereby incorporated by reference. For a discussion of the properties of various foamed plastics and of the foaming process itself, reference is made to the article entitled "Cellular Plastics", in the Encyclopaedia of Polymer Science and Engineering, Vol 3, pp. 1-59 (1985), the disclosure of which is herein incorporated by reference.
Conventional rotomolding has been used mainly for the production of large solid, hollow articles with a very uniform wall thickness, hereinafter referred to as "conventional rotomolding". However, recent developments in rotational foam molding have demonstrated that the technology may also be used to make low-density, fine-cell plastic foam articles ("rotational foam molding"). A foamed component may advantageously be used to fill up the hollow rotomolded product to enhance its physical and mechanical performance for various end applications.
By introducing a physical or a chemical blowing agent (CBA) into a polymer matrix, foam structures can be produced having low specific weight, low thermal conductivity and energy absorption properties useful in applications in the packing industry, automobile parts and construction industries.
Basically, the foaming process consists of three steps: (i) the creation of small discontinuities or cells in plastic melts; (ii) growth of these cells to a desired volume; and (iii) stabilization of the resulting foam structure. Physical blowing agents such as CO.sub.2, N.sub.2, or low-boiling liquids and chemical blowing agents (which produce carbon dioxide or other gases by chemical reaction) are used for creating cells in plastic melts. Gases or volatile liquids are generally injected directly into the polymer melts at high temperature and pressure. Chemical blowing agents (CBAs), by contrast, are blended with the plastic pellets or powders and undergo gas-forming thermal decomposition during the melting/molding process.
The most widely used commercial products are polystyrene (PS) foams, particularly in the field of packing and food containers. Polyethylene (PE) foams have also been produced by various processing methods. Polyethylene resin is widely in current use in rotational molding applications, both in foamable and in non-foamable resin compositions. Reference is made, for example, to the polyethylene-based compositions for rotational molding which are disclosed in U.S. Pat. Nos. 5,366,675 and 5,532,282 issued to Needham.
For many applications, however, the respective service temperatures of about 100.degree. C. and 125.degree. C., respectively, for PS and PE foams are too low. Their use as structural materials is also limited by reason of the low impact strength of PS foams and the low modulus of PE foams. In comparison with PS and PE, polypropylene (PP) has a much higher melting temperature, approximately 160.degree. C. PP-based materials also exhibit a much higher impact strength than PS and a higher stiffness than PE, recommending PP as a good candidate for the production of high-strength foam structures.
Although PE is much more amenable to known foaming procedures than is PP, PP foams are preferable for those applications where stiffness, chemical resistance, good heat insulation, sound deadening and higher end-use temperatures are required. Examples include the automotive industry ("under-the-hood" high service temperature parts, interior, and cushioning applications) and insulation for domestic and commercial hot water and air conditioning pipes.
Unfortunately, the melt strength of PP decreases very quickly with an increase in temperature above its melting temperatures, leading to a narrow processing "window" and attendant great difficulties in foam processing. Quite typically, attempts to foam PP result in very small cell population densities and large average cell sizes and/or lack of uniform structure. Consequently, there are currently very few practical PP foam applications.
Several processing technologies are being developed for the production of PP foams, including extruded PP foams, injection molded PP foams and the processing of foams from expandible PP beads. For example, U.S. Pat. No. 4,940,736 (Alteepping et al.) discloses a composition of 70-90 wt % of low-viscosity polypropylene component having a melt viscosity of less than 2.times.10.sup.3 poise and 30-10 wt % of high viscosity polypropylene component (greater than 2.5.times.10.sup.3 poise). Using CFCs as a physical blowing agent, this mixture of polypropylenes was extruded to produce a fine-cell polypropylene foam.
In an article appearing in the March 1991 edition of Plastics Engineering, entitled "Novel Polypropylene for Foaming in Conventional Equipment", the authors N. B. Bradley and E. M. Philips discuss high-melt strength polypropylene homopolymers.
In a subsequent article appearing in the Sep. 20, 1995 edition of the Proceedings of Scotland Polyolefins Conference, entitled "A New Family of High Melt Strength Polypropylene Copolymers for Extruded Low Density Foam Applications", V. P. Bavaro describes long-chain branched high melt-strength propylene-ethylene copolymers for extruded low density foam applications.
However, we found the information provided in prior descriptions of extrusion or injection molding of PP homopolymers and copolymers not to be directly pertinent or instructive for the formulation of useful foamable polypropylene-based resin compositions for rotational molding.
In the rotational foam molding process, a foamable plastic blend or powdered plastic composition is put into a closed mold and exposed to heat while the mold is rotated. As a result, the foamable plastic materials become sticky, adhere to the inner surface of the mold layer by layer and ultimately sinter to form a uniform liquid layer.
At the time when the temperature of the melt reaches the onset decomposition temperature of the CBA, the CBA particles begin to decompose and liberate gas thereby initiating the foaming processs of the plastic. Once the predetermined time for the heating cycle has elapsed, the process proceeds with the cooling cycle. After sufficient cooling, rotational movement of the mold is stopped and the finished part is removed. The foamed material comprises a continuous phase (polymeric matrix) with a discontinuous gas phase distributed through the matrix. As noted above, the foaming process involves the creation of small discontinuities or cells, the growth of the cells to a desired volume and stabilization of the resultant foam structure.
Quite unlike the extrusion molding and the injection molding processes, in the process of rotational molding the predominant force leading to polymer flow within a mold is simply that of gravity. The gravitational force imposes very low shear stresses and shear rates, typically in the range of 10.sup.-2 -10.sup.-6 s.sup.-1. Shear rates characterizing extrusion and injection molding processes are typically in the range of 10.sup.2 -10.sup.3 s.sup.-1, and 10.sup.3 -10.sup.4 s.sup.-1, respectively. It is therefore not surprising that quite different process parameters come into play in preparing fine-celled PP foam by rotational foam molding.